Degradation isomerisation

C02 elimination and the substitution with water were the most exothermic degradation reactions and this confirmed the observation of C02 evolved during degradation. Isomerisation reactions were also investigated and assessed for the possible reaction pathway exothermicity. The calculations did not try and assess the transition states or barriers along the reaction pathways. 

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

Studies have recently become available concerning the kinetics and mechanism of the isomerisation, in dilute aqueous solution, of dihydrotriazines of this type (e.g. 4,6-diamino-l-(3,5-dichlorophenyl)-l,2-dihydro-2,2-dimethyl-l,3,5-triazine), and of their degradation to substituted biguanides 683). 

Table 21.3 Reaction products from isomerisation and degradation of (Z)-3-hexenal...

More recently, the degradation of a-pinene by Pseudomonas jluorescens NCIMB 11671 was described [97,98]. A novel pathway for the microbial breakdown of a-pinene (119) was proposed, Fig. (23). The attack is initiated by enzymatic oxygenation of the 1,2-double bond to form the epoxide (127). This epoxide then undergoes rapid rearrangement to produce a novel diunsaturated aldehyde, occurring as two isomeric forms. The primary product of the reaction (Z)-2-methyl-5-isopropylhexa-2,5-dien-l-al (trivial name isonovalal) (128) can undergo chemical isomerisation to the -form (novalal) (129). Isonovalal, the native form of... 

Table 3.3 Effect of high pressure on isomerisation and degradation of lactose in alkaline media for 3 h at 60 °Cin...

Additionally, it has been shown that photolytic isomerisation/degradation of HBCDs on indoor dust occurs in a fashion that both reduces the concentration of ZHBCDs and effects a marked shift from y- to a-HBCD (Harrad, Abdallah, and Covaci, 2009). 

Isomerisation is the process of conversion of a dmg into its optical or geometric isomers. Since the various isomers of a dmg are frequently of different activity, such a conversion may be regarded as a form of degradation, often resulting in a serious loss of therapeutic activity. For example, the appreciable loss of activity of solutions of adrenaline at low pH has been attributed to racemisation - the conversion of the therapeutically active form, in this case the levorotary form, into its less-active isomer. 

C, Synthesis.—Atisine and Veatchine Types. Zalkow and co-workers developed syntheses of intermediates potentially transformable into atisine-type alkaloids, starting with podocarpic acid. The general approach is illustrated by a synthesis" of an ajaconine degradation product. Methyl O-methyl-7-keto-podocarpate (56) was reduced to the diol, which was converted by Birch reduction to dienone (57). The diene diol diacetate from this was converted to the 7,8-epoxide. Boron trifluoride converted this to the non-conjugated enone (58) which isomerised and... 

Fig. 8.6 A proposed mechanism for the degradation and isomerisation of carotenoids in processed and stored vegetables.

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